Zirconium oxide-based composite material may be used in the dental sector in the production of bridges and crowns, for example in the production of dental implants, in the production of medical components such as spinal implants, but also in general in areas in which a technical ceramic with damage-free hard machinability is required, for example in machining operations such as cutting, milling, and drilling.
Ceramic materials have advantages over conventional metallic materials in the dental market due to their chemical resistance, mechanical and physical properties, and optical properties, which provide excellent esthetics.
The general trend in dental ceramics is toward “full ceramic systems.” However, at the present time ceramics are still frequently applied as veneers on metallic frameworks. Dental ceramics may be classified based on their production method and their crystalline phase.
Metal-ceramic systems have been in existence since 1960. A veneer ceramic is applied to a metal framework in order to obtain an esthetically acceptable restoration which resembles the natural tooth. Typical veneer materials are composed of feldspathic glasses, and are usually based on leucite crystal. The addition of leucite crystals (KAlSi2O6) into the feldspathic glass structure results in optimal properties with regard to the coefficients of thermal expansion of the framework and the veneer. Leucite crystals are formed by incongruent melting of natural feldspar at temperatures between 1150 and 1530° C. The coefficient of thermal expansion may be controlled in a targeted manner and adapted to the metallic framework by varying the leucite crystal content. The typical leucite crystal content in feldspathic glass is commonly between 15 and 25 vol.-%. The coefficient of thermal expansion is thus lower than that of the metal, and the applied veneer is placed under pressure. In the classical process, veneer ceramics are sintered under vacuum to reduce the porosity in the final product. On account of the glass phase, the mechanical properties of the leucite crystal-based glasses (also referred to as dental porcelain) are the lowest of all ceramic materials used in dentistry. As of 2005, 50% of all dental restorations were produced using metal-ceramic systems.
Full ceramic systems are free of metal, and have been available for 30 years. The process technology is undergoing continuous development (hot pressing, slip casting, CAD/CAM machining, for example). The main difference from the metal-ceramic systems is a much higher proportion of the crystalline phase, which may be between 35 and 100 vol.-%. Although the mechanical properties are improved, the opacity is increased, which is disadvantageous with regard to the required esthetics.
There are a number of factors that have an influence on the longevity of the full ceramic systems, such as the oral environment, fluctuating pH values from acidic to basic, cyclical load, and extreme load peaks during chewing. Full ceramic systems having higher proportions of the glass phase frequently exhibit stress corrosion cracking as the cause of failure. Due to the hydrothermal aging of Y-TZP ceramics (100 vol.-% crystalline phase, Y-stabilized tetragonal zirconium oxide) at low temperatures, testing according to standards is required in which the longevity in the human environment and under cyclical load is to be assessed.
Full ceramic systems are classified primarily based on the production method (hot pressing, dry pressing and sintering, slip casting, CAD/CAM machining, for example). In hot pressing, leucite crystal-based glasses having a crystalline phase proportion between 35 and 45 vol.-% are initially used. The mechanical properties are higher than those of the leucite crystal-based glasses of the metal-ceramic systems by a factor of 2. Repeated heating may facilitate the leucite crystallization and may result in greater strength.
A novel glass ceramic is presently used for the hot pressing. The material is composed of a lithium disilicate-based glass having a crystalline phase proportion of 65 vol.-%. X-ray analyses have identified lithium disilicate (Li2Si2O5) in addition to further crystal phases such as lithium metasilicate (Li2SiO3) and cristobalite (SiO2). The mechanical properties are once again higher than those of the leucite crystal-based glasses by a factor of 2.
The dry pressing and sintering of full ceramic systems has been used since the early 1990s. The production is carried out with computer assistance, and takes into account the sintering shrinkage of the compacted blank during sintering. Ceramics based on aluminum oxide and zirconium oxide (100 vol.-% crystalline phase proportion) are used as the framework material, upon which a veneer made of glass ceramic is additionally applied. Aluminum oxide ceramics are characterized by a flexural strength of approximately 600 MPa and excellent in vivo behavior.
Slip casting has been used since the 1990s. In the process, a porous green body is produced from crystalline phases by means of slip casting, subsequently sintered, and infiltrated with a lanthanum-based glass.
The following glass ceramics are available on the dental market: aluminum oxide (Al2O3), spinel (MgAl2O4), and 12Ce-TZP/Al2O3 composite. The glass-infiltrated aluminum oxide has mechanical properties comparable to those of lithium disilicate-based glass ceramic, but has a minimally higher opacity. The glass-infiltrated spinel has much higher translucence and comparable mechanical properties compared to lithium disilicate-based glass ceramic. The glass-infiltrated zirconium oxide/aluminum oxide composite exhibits the highest strength and fracture toughness of all slip-cast dental ceramics.
The computer-controlled CAD/CAM machining of ceramic blocks and blanks has been carried out since the early 1970s, and was introduced by Duret. At that time, the machining was performed on dense-sintered blanks. Presently, operations are carried out primarily using pre-sintered blanks.
Glass ceramic is suitable for CAD/CAM machining in the dense-sintered state due to its very good machinability. Typical mica crystal-based glasses were formerly used on account of the ideal machinability. At the present time, feldspathic glasses containing sanidine, leucite, or lithium disilicate crystals are used. However, the CAD/CAM machining on dense-sintered glass ceramics shows significant tool wear. Surface defects may adversely affect the in vivo behavior.
In general, glass ceramics have very good machinability. Microcracks develop along the phase boundaries due to differing coefficients of thermal expansion of the crystal and the glass matrix during cooling. In addition, the crystalline phases have very good cleavability along the longitudinal orientation (primarily mica along the crystallographic (001) plane). The crystal phases should have no preferred orientation; i.e., they should be isotropically distributed in the glass structure. A crack introduced by a tool runs along the cleavage plane or also along the phase boundary between the crystal and the glass matrix. As a result, the crack is continuously deflected during the machining, and only small areas of the surface are broken out from the workpiece.
Since 2001, CAD-CAM machining has been carried out on pre-sintered zirconium oxide blanks. The machining is easier and quicker, and shows lower tool wear than when dense-sintered blocks had to be machined. However, the finished workpieces must subsequently be dense-sintered. Fluctuations in the sintering shrinkage, accompanied by dimensional deviations and touch-up operations performed manually by dental technicians, result in an increased risk of damage to the zirconium oxide.
Thus far, zirconium oxide as the framework material has the best mechanical properties. However, due to phase transformation of the tetragonal zirconium oxide phase, cracks frequently occur at the interface between the framework and the veneer on account of the additional required veneer ceramic. Several previously published 3-year and 5-year in vivo studies have already been conducted quite some time ago.
The studies concluded that there is an excellent success rate, but with a low survival rate with complications such as incidence of dental caries or chipping of the veneer. The current development trend is clearly toward zirconium oxide/aluminum oxide composite materials, with the objectives of improving the hydrothermal aging resistance and the mechanical properties.